The present invention relates to a plasma reactor.
Where a plasma reactor is used to deposit a film atop a substrate, unwanted deposition of the material on passive surfaces within the reactor is a common result. In particular, where loose material is deposited, which may subsequently be transported back into the exposed areas of a device being processed, variable and undesired device characteristics may result.
For the plasma deposition of silicon nitride films, it is sometimes advantageous to use high flows (approximately 100 cc/min) of the pure reactant gases, SiH.sub.4 and NH.sub.3. These high reactant flows allow a rapid (300 .ANG./min) deposition rate of silicon nitride on heated (300.degree. C.) substrates while maintaining desirable film properties such as compressive film stress. It has been found, however, that a loose powder may form on the cooled lid and sidewalls of the reactor when these high reactant flows are used. This powder formation is undesirable as it is a source of particulates which can degrade the mechanical integrity of the silicon nitride deposit on the heated substrates. This is an outstanding problem in every plasma-assisted chemical vapor deposition process. It is of particular concern in nitride deposition since plasma CVD is the most convenient way to deposit nitride.
A schematic of the radial flow plasma reactor concept (U.S. Pat. No. 3,757,733) is shown in FIG. 1. Reactant and diluent gases are introduced below the substrated plate, flow radially inward between the rf electrode and the substrate holder, and are pumped out of the reactor. The plasma which is excited between the rf electrode and the substrate holder allows the deposition of silicon nitride or other compounds at a relatively low substrate temperature (approximately 300.degree. C).
Using a 24-inch reactor, approximately 60 watts of rf power are required to deposit a uniform silicon nitride film over the entire surface of the heated substrate holder. This power level can result in deposited film properties, such as low density and tensile stress, which may be undesirable. Higher power levels (several hundred watts), however, result in a plasma glow beneath the substrate plate. This glow results in premature activation of the reactants, resulting in a thich deposit near the periphery of the substrate holder and a thin deposit near its center.
A method for suppressing the plasma glow beneath the substrate plate and increasing the usable rf power level was described in U.S. Pat. No. 4,033,287. A schematic of this reactor arrangement is shown in FIG. 2. The reactant and diluent gases are piped into a gas distribution ring located on the periphery of the substrate holder. It was demonstrated that the useful power level could be increased to several hundred watts, allowing control of film properties over the entire area of the substrate holder.
Thus, it is an object of the present invention to provide a method for operating a plasma reactor for deposition of materials on substrates within the reactor, such that material is not also deposited on the sidewalls of the reactor.
It is a further object of the present invention to provide an improved plasma reactor for deposition of silicon nitride films, such that high flows of the reagent gases may be used without depositing granular silicon nitride on the sidewalls.